CREATE [UNIQUE {ON FIRST {COLUMN | n_unique_column COLUMNS}} INDEX ON table_name (column_name1, column_name2 ...);
I would use the first (simple) syntax and just throw an error if the user tries to skip a column on the UNIQUE clause.
Seems, second option looks as more natural extension of CREATE UNIQUE INDEX
True, but it's awefully verbose. :( And...
It surprised me that you can INCLUDE extra columns on non-UNIQUE indexes, since you could just add them as regular indexed columns for the same effect. It looks like when you do that in SQL Server, the extra columns are only stored on btree leaf pages and so can't be used for searching or ordering. I don't know how useful that is or if we would ever want it... but I just wanted to note that difference, and that the proposed UNIQUE ON FIRST n COLUMNS syntax and catalog change can't express that.
... we might want to support INCLUDE at some point. It enhances covering scans without bloating the heck out of the btree. (I'm not sure if it would help other index types...) So it seems like a bad idea to preclude that.
I don't see that UNIQUE ON FIRST precludes also supporting INCLUDE. Presumably we could do either
CREATE INDEX ... ON table (f1, f2, f3) UNIQUE(f1, f2) INCLUDE(f4); or CREATE UNIQUE ON FIRST 2 COLUMNS INDEX ... ON table (f1, f2, f3) INCLUDE(f4);
Personally, I find the first form easier to read.
Why not normal syntax with optional INCLUDE ?
CREATE UNIQUE INDEX ON table (f1,f2,f3) INCLUDE (f4)
That's not the same thing. Then you're including f3 in the unique constraint, which you may not want for uniqueness purposes, but may want in the index for sorting. But then saying that, if f1 and f2 are unique, you'd only get 1 value of f3 for each combination of f1 and f2, so sorting probably isn't useful. You might as well only INCLUDE f3 rather than have it in the multi-column index for sorting. So to adjust your example:
CREATE UNIQUE INDEX ON table (f1,f2) INCLUDE (f3, f4);
Is there a scenario anyone can think of where f3 here:
CREATE INDEX ... ON table (f1, f2, f3) UNIQUE(f1, f2) INCLUDE(f4);
would be advantageous outside of INCLUDE?
Out of curiosity, why is this only being discussed for unique indexes? What if you want additional columns included on non-unique indexes?
